were supported by fellowships from FNRS-FRIA (Fonds pour la Formation la Recherche dans lIndustrie et dans lAgriculture) (Grants 1

were supported by fellowships from FNRS-FRIA (Fonds pour la Formation la Recherche dans lIndustrie et dans lAgriculture) (Grants 1.E100.14 and 1.E082.14). Footnotes Competing interest statement: B.J.V.d.E. SD of duplicates). NS: for medium with tryptophan versus without tryptophan (two-way ANOVA). ((mean + SD of duplicates). 0.0001 for 680C91 and time parameters (two-way ANOVA). (and transcript in the liver remained constant (Fig. 2= 20). Four mice were killed at each indicated time point, and TDO protein levels in the liver were evaluated by western blot and normalized to vinculin. One representative mouse out of four is illustrated for each time point ( 0.05; ** 0.01; *** 0.001; **** 0.0001 for different time points versus initial situation (Unpaired test with Welch correction). This experiment was performed independently three times. (mRNA levels were measured by quantitative RT-PCR in the liver and normalized to actin. Statistical analyses were performed as in and and and and (HEK293 hTDO cl119) were transiently transfected with the dominant-negative forms of cullins (CULDN) as indicated or with empty vector (pcDNA3). Cells were initially cultured in medium containing tryptophan (start) then incubated for the indicated times in medium containing cycloheximide (50 g/mL) in the presence or the absence Ginkgolide C of tryptophan (80 M). Protein levels of TDO and CULDN were evaluated by western blot with anti-TDO mAb clone III and with an anti-FLAG antibody, respectively. Experiments were performed independently three times for CUL1 and CUL3 and two Ginkgolide C times for the other cullins. CRLs are multiprotein complexes built around a cullin scaffold, whose C terminus recruits a catalytic small RING protein (RBX1 or 2) that interacts with E2 enzymes, while the N terminus interacts with a receptor responsible for substrate recognition, such as an F-box protein (25). There are several cullin variants, including CUL1, CUL3, CUL4A, CUL4B, and CUL5. To identify which CRL was involved in TDO polyubiquitination, we used C-terminally truncated forms of cullins, which are inactive as they are unable to interact with RBX proteins and E2 enzymes but can still bind the substrate receptor and therefore act as dominant-negative cullins (26). We transiently expressed the dominant-negative cullins in human embryonic kidney cells HEK293-EBNA stably transfected with TDO. We observed that transfection of the dominant-negative CUL1 increased the half-life of TDO in the absence of tryptophan compared to cells transfected with the empty vector (Fig. 4and and shows the kynurenine and tryptophan concentrations measured by HPLC in the supernatants. Mean + SD of triplicates. 0.05 for medium with alpha-methyl-tryptophan versus without alpha-methyl-tryptophan (two-way ANOVA). (= 32). Four mice were killed at each indicated time point to evaluate TDO protein levels in the liver by western blot. One representative mouse Ginkgolide C of four is illustrated for each time point ( 0.05; ** 0.01 for each time point versus initial situation (unpaired test with LKB1 Welch correction). One representative out of three independent experiments is shown. ( 0.0001 for TDO-mutated proteins versus TDO WT (unpaired test with Welch correction). (identify amino acids whose mutation stabilizes TDO in the absence of tryptophan. These amino acids represent the likely degron that is masked by tryptophan binding to the exosite. (and and Fig. 5for 10 min. Protein concentrations were measured by the bicinchoninic acid assay (BCA) (#23225, Pierce). Then, proteins (15 or 20 g) were heated at 70 C for 10 min with NuPAGE LDS (lithium dodecyl sulfate) Sample Buffer (NP0007, Thermo Fisher) and NuPAGE Sample Reducing Agent (NP0009, Thermo.